Theming is a way to replace a set of ...
Theming is a way to replace a set of views with another without the need of touching the original view rendering code. You can use theming to systematically change the look and feel of an application.
To use theming, you should configure the [[yii\base\View::theme|theme]] property of the
view application component. The property configures a [[yii\base\Theme]] object which governs how view files are being replaced. You should mainly specify the following properties of [[yii\base\Theme]]: